1158

Item 1158

DESIGN: Dragonfly ~ Rotor - Hub - Head

Initially, for the prototype a metal head could be made, similar to the wood mockup, and screwed onto the final design of Jesus nut.

Mockup Pictures:

Drawing:

Loads from Flapping Hinge:

The large force is inward toward the mast. (compression from centrifugal and thrust)
A smaller force is upward. (from thrust)
A much smaller force is downward (rotor at rest or extreme downward flap.)
A much smaller force might be outward (if the tie-bars accept compression)

Note:

Over a 3.625" diameter * 24" (approx.) long mandrel roll up unidirectional prepreg cloth, which has the threads running in the 20" direction.
Roll until the OD is 4.375"
Slit the cylinder from one end so that there are 6 equal strips approximately 15" long.
Insert the mail mold over the mandrel, to position the 6 tongues (arms).
vacuum bag and cure.
wrap a1" (approx.) unidirectional tape around the cured piece, at just below the start of the splits until the OD is 4.75"
Cure.

Build the arms up from small rectangular pultruded rods over a foam core. Then wrap the assemblage in diagonal ply cloth.

Material:

If the zig-zak shape above is made out of 1/4" thick aluminum, it will weight 8 lbs for the three arms. It must, eventually be made in composites.

Notes:

Some lag could be absorbed in the three 'arms' between the center of the head and the teetering/flapping hinges.
The flap bearings might have to have their own holder, so that the outer yoke can be placed between the head's two arms.

The loading is mainly compressive and there is currently more clearance over the center of the hub then part way out the arms. Therefor consider making the hub predominately a 1/2" deep cap, which is higher in the middle. Having the arms climb slightly, as the advance to the mast, will be more inline with the compressive force.

Amount of hub material at teetering axle:1

The axle has a 3/8" diameter. This is 0.1104 square inches.

A cross section of the area directly above the axle is 3/8' wide by 3/8" high on the outside and 7/16" high on the inside. This is 0.1523

Attachment to mast:

Consider bonding the mast to a "cylindrical nut'. The interior would be partially threaded and the exterior would be knurled.
Alternatively, thread the composite mast with a very course thread.
The threads on the two masts should probably be opposite hand, so that operation will not want to unscrew the masts.

Thermal Expansion:

Coefficient of Thermal Expansion - Metal

		10e -6
	Aluminum	13.1
	Titanium	5.3
	Stainless steel	9.6
	Steel	6.5

Coefficient of Thermal Expansion - Composite

Honeycomb will change its dimensions slightly when subject to a change in temperature. The change in dimensions as a function of temperature is determined by the substrate material. Coefficients of thermal expansion in the thickness direction for various honeycomb materials are as follows:

Honeycomb Core		Coefficient of Thermal Expansion (inch/inch – ºF)
CR III, CR-PAA, 5052, 5056, ACG Aluminum		13.2 x 10–6
HRP, HFT, HRH-327 Fiberglass		8.2 x 10–6
HRH-10, HRH-310, HRH-78 Nomex		19.4 x 10–6
HRH-49 Kevlar		2.7 x 10–6
HFT-G Carbon	2.0 x 10–6

Will the difference in expansion eventually cause the screw thread (or the knurl) to become sloppy?

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Last Revised: November 21, 2003